This invention relates to roll-over valves which are used in the fuel systems of automotive vehicles.
The emission of volatile fuel vapors to atmosphere is considered to be such a significant source of environmental pollution in the United States that laws and government regulations have been imposed on automobile manufacturers to limit the amount of such vapors emitted from automobile fuel systems. Today's automotive vehicles are equipped with evaporative emission control systems whose purpose is to prevent the escape of volatile fuel vapors into the atmosphere.
Ideally, fuel systems of automotive vehicles should be completely closed to prevent vapor escape, but practical operational considerations must be taken into account, and therefore the fuel tank and related parts of the fuel system are not truly completely closed; rather, certain valving functions are associated with them. For example, extremes of heat and cold will affect the pressure in the tank's headspace giving rise to the need for venting and vacuum pressure relief functions for limiting positive and negative tank pressure respectively. Likewise, as fuel from the tank is consumed by the vehicle's engine, air must be introduced into the tank's headspace to limit the magnitude of negative tank pressure. Such venting and vacuum relief functions are performed through valves.
A typical evaporative emission control system in an automotive vehicle comprises a vapor collection canister which is connected to the headspace of the fuel tank by a conduit and which contains a medium capable of adsorbing volatile fuel vapors. When the vapor pressure in the tank's headspace rises above a certain positive level, the tank's headspace is vented to the canister, and vapors flow from the tank into the canister where they are collected. In this way the uncontrolled escape of vapors to atmosphere is avoided, yet the tank is vented as required. Similarly, an increase in the magnitude of negative tank headspace pressure, i.e. vacuum, is relieved by reverse flow into the tank. A combination vent/relief valve mechanism may be included in the conduit between the tank and canister to perform these functions whenever they are required. Under engine operating conditions the canister will be purged by the engine vacuum which will deliver controlled volumes of collected fuel vapors to the engine for combustion.
The various valves associated with the fuel tank, and any other conduits connected to it, may constitute potential sources of liquid fuel leakage in certain automobile accidents, especially roll-over type accidents. Such leakage of liquid fuel could cause even more dire consequences than the roll-over itself. Concern about roll-over-caused fuel leakage is also the subject of regulation, and for compliance, roll-over shut-offs have been incorporated into fuel tank conduits and valves to prevent the escape of liquid fuel in the event of a roll-over type accident.
A roll-over shut-off is normally open but closes in response to a predetermined amount of tipping indicative of vehicle roll-over. The definition of roll-over is not limited to the vehicle becoming completely upside down; it includes a range of lesser extremes. If full roll-over is defined as 180 degrees of inversion from upright, partial roll-overs are defined between this maximum and some lesser selected angle which is chosen in accordance with the particular design considerations. In other words, a roll-over shut-off allows for a certain amount of tipping from vertical, but closes when the allowed amount of tipping is exceeded.
Certain types of roll-over shut-offs also possess the ability to close when immersed in liquid fuel. For example, when a roll-over shut-off of this type exercises control of a tank's vapor vent, not only will it close the vent upon roll-over, but if the liquid fuel rises to the level of the vent in the absence of roll-over, the shut-off will also close the vent line to prevent liquid fuel from passing through the vent.
A roll-over shut-off has a mechanism which is attitude-responsive whereby in response to tipping from a nominal upright position beyond a predetermined permissible amount, the shut-off c1oses the conduit or valve with which it is associated. The most common form of attitude-responsive mechanism comprises a weight which controls a shut-off element. The weight holds the element open so long as the mechanism is not tipped beyond the permissible amount, but pushes the shut-off element closed against a seat once the mechanism has been tipped into the roll-over range. The amount of tipping required to close the shut-off against the seat is a function of several factors such as the mass, shape, and density of the weight, and any additional active components, such as springs, or caged balls, for example
So long as the automotive vehicle is operating in the usual manner over relatively flat terrain, or up and down hills, the roll-over shut-off element is held open by the mechanism, but in the event of a roll-over type accident, the mechanism closes the shut-off element to prevent liquid fuel leakage from the fuel tank via the associated conduit or valve.
Examples of various types of fuel system roll-over valves are shown in a number of U.S. patents. Included among these is Applicant's own U.S. Pat. No. 4,646,772.
Additional concerns about the escape of volatile fuel vapors to the atmosphere has prompted further governmental regulation in the United States addressing the containment of vapor during filling of automobile fuel tanks at service stations. Consequently, additional complexity will have to be introduced into automobiles' evaporative emission control systems and it is possible that multiple roll-over shut-offs, and/or more complex roll-over shut-offs may be required in future vehicles.
While the present invention in its generic aspects is not limited to the particular manner of the roll-over shut-off's use in the fuel system, certain uses, such as in a vent between the tank and the canister of the evaporative emission control system, impose more severe constraints on the shut-off than others, and this is where the advantages of the invention are most appreciated. The invention attains a very significant improvement in those uses where it is important that the force required to re-open a closed shut-off be minimized.
In regular operation of an automotive vehicle having a roll-over shut-off in the vent line to the evaporative emission control system, it may happen that the roll-over shut-off element closes under certain conditions unrelated to and not involving vehicle roll-over. In one example, a roll-over vent valve assembly which is mounted in a sealed manner in an opening in the top wall of the fuel tank may have its interior exposed to the action of liquid fuel in the tank, either fuel slosh within the tank, or some degree of immersion in the fuel due to the fuel level approaching the top of the tank. While the associated mechanism which controls the shut-off element will, per se, seek to keep the shut-off element open so long as the valve attitude remains within the allowable range about nominal upright, the buoyancy of fuel may temporarily override the controlling influence of the mechanism and cause the shut-off element to close.
In the case of momentary fuel splash, such closures should be only temporary, and the mechanism should promptly regain control to re-open the shut-off element so that there are no adverse consequences on tank headspace pressure. However, if the tank headspace is being vented at the time that fuel splash closes the shut-off element, there would exist pressure differential between the tank and canister. This pressure differential acts on the closed shut-off element, adding to the force required to re-open it, and possibly rendering the mechanism alone unable to re-open it. Therefore, certain roll-over shut-offs which close during venting do so with the consequence that a noticeably higher re-opening force must be exerted in order to overcome the additional component of differential pressure force acting on the closed shut-off element, as well as any further component due to friction between the closed shut-off element and the seat against which the shut-off element is closed. At times then, the shut-off element stays closed when it should re-open. This condition is commonly referred to as "corking".
If the shut-off element is to be intentionally closed when the liquid fuel rises to a certain level near the top of the tank, the attitude-responsive mechanism which operates the shut-off element is endowed with a certain characteristic of buoyancy via which closure of the shut-off element is accomplished. When the fuel level acting on the weight subsequently drops below that at which the shut-off element was buoyed closed, the shut-off element should re-open. Conditions, such as pressure differential, acting on the closed shut-off in the direction of closure will add to the re-opening force requirement and corking of the shut-off element may result here too.
One way to minimize the re-opening force requirement is to reduce the size of the opening controlled by the shut-off element so that for a given differential pressure there will be a lesser component of pressure differential force which must be overcome, and likely a lower frictional force component as well. Unfortunately, this reduction in opening size restricts the flow capacity of the conduit between the canister and the fuel tank and may pose a problem with evaporative emission system compliance and/or operation.
An alternative solution is to increase the mass of the weight which acts to open the closed shut-off, but the amount of material and size of the weight will have to be increased in an area where only limited space is available. Accordingly, this alternate solution may not be feasible in many instances.
In certain generic aspects the present invention relates to an improvement in a roll-over vent valve which, for given physical characteristics of the attitude-responsive mechanism acting on the roll-over shut-off element and for given size of opening controlled by the shut-off element, attenuates the force required to re-open the closed shut-off element compared to valves presently in production. Stated another way, a given force acting against a given pressure differential to re-open the closed shut-off, can re-open a larger opening than valves presently in commercial use.
The present invention achieves this result by having the attitude-responsive or liquid level shut-off mechanism act on the closed shut-off element initially along a limited circumferential extent of its perimeter. This causes the re-opening force to be concentrated along a mere fraction of the full circumferential extent of sealing contact of the shut-off element with the seat; the action is like a mechanical advantage or force amplification. The result is a partial unseating of the shut-off element along a limited circumferential segment of the seat. By then continuing the force progressively around the shut-off element, it is finally completely unseated. The opening/closing characteristics are determined by the design of the attitude-responsive or liquid level shut-off mechanism in relation to the shut-off element and seat.
Several embodiments of the invention are disclosed. In one embodiment, the shut-off element is in the form of a thin, somewhat pliable, circular disc which is captured within a multi-fingered retainer attached to a float body. The retainer and float body constitute a weight for controlling the disc. A helical spring exerts a force on the weight in the direction of closure and the combined effects of weight and spring define the characteristics of the attitude-responsive mechanism.
The retainer and float body cooperatively form a circular cylindrical zone of confinement for the disc relative to the seat. The outside diameter (O.D.) of the disc is greater than that of the seat but the diameter of the zone of confinement is slightly larger so that the disc is capable of a limited amount of radial displacement within the confinement zone. The disc is proportioned relative to the seat such that at any degree of radial displacement of the disc within the zone, a portion of the disc is always in full axial confrontation to the seat. Hence, when the disc is intended to be closed against the seat, such closure will be assured. Closure occurs by the weight moving toward the seat.
The fingers are spaced circumferentially around the retainer and extend axially past the edge of the disc. The distal ends of the fingers project radially inwardly to overlie the marginal edge of the disc, and it is via these radially inwardly extending portions of the fingers that re-opening force is imparted to the disc when the weight moves away from the seat. The fingers have differing axial lengths so that the fingers become effective in succession on the disc, sequentially around the disc's perimeter. By making the disc slightly pliable, it both forms a good seal for closure, and is conveniently peeled away from the seat for re-opening.
In another embodiment, one or more points on the edge of the disc are tethered by one or more thin ribbons, or cords, to the weight. These cords perform the same unseating function as the fingers. Hence, when the weight moves away from the seat, the cord, or cords, are tensioned and pulled in succession at different points along the edge of the disc thereby unseating the disc.
Further details, features, and advantages of the invention will be seen in the ensuing detailed description, the accompanying claims, and the drawings. The drawings disclose a preferred embodiment of the invention according to the best mode contemplated at the present time in carrying out the invention.